This invention relates to a neutralization signal developing device for an echo suppressor, which is required for data transmission over a telephone transmission line having four-wire circuits.
A commercial telephone transmission line generally consists of a two-wire circuit in the vicinity of each subscriber and a four-wire circuit for the long-distance link, with a hybrid circuit connecting them. At the hybrid circuit, however, echoes are produced in a transmission line when an impedance discontinuity or mismatch exists. The annoying effects of echoes can be minimized by the use of an echo suppressor which is situated near one end of a four-wire circuit to disable the outgoing, or echo return, path from a particular subscriber when signals from the far-end subscriber appear on the incoming path. This can be achieved by monitoring the levels of speech signals on the outgoing and incoming paths and by inserting a comparatively large loss into the outgoing path to reduce the level of echoes only when the signal level on the incoming path is higher than that of the outgoing path.
Meanwhile, when data transmission is carried out through a four-wire circuit connected to such an echo suppressor using modulators and demodulators (MODEMs), which are installed at both terminal stations, the coincident transmission of data from the MODEMs occurs frequently. If the signal level of data on the outgoing path is lower than that of data on the incoming path, data from at least one MODEM cannot be transmitted because of the above-mentioned operation of said echo suppressor. For this reason, means responsive to an identification signal, which is given from the MODEM immediately before data transmission, for neutralizing the operation of the echo suppressor, is indispensable for the transmission of data from the MODEMs. In this connection, the C.C.I.T.T. (see International Telecommunication Union, Sixth Plenary Assembly, Orange Book, Volume III-1, 1977, Recommendation G-161, pp. 93-94) has recommended that said neutralizing means should satisfy the following requirements:
(1) An identification signal, i.e., a disabling tone preceding the transmission of data shall be continuously emitted 400 milliseconds at a frequency of 2,100.+-.15 Hz; (2) in view of the characteristics of the transmission line, neutralizing means shall operate under the effect of the identification signal having a frequency of 2,100.+-.21 Hz; and (3) said means shall operate in 300.+-.100 milliseconds after the reception of the identification signal.
An example of such a neutralizer satisfying the requirements of the C.C.I.T.T. recommendation includes a structure using an analog band-pass filter as illustrated in FIG. 2 of a paper titled "629 Tone Disabler" published in Wescom, Inc., Circuit Description/Installation Series, Section 629-101/3, March issue, 1972. There is a disadvantage, however, that this analog neutralizer has a complicated structure. Meanwhile, a digital neutralizer to solve this problem has been proposed in U.S. Pat. No. 3,935,403 issued to Penicand et al. In this device, the detection of the identification signal is carried out by counting the number of variations (zero-crossings) in sign of the sampled signal during a given period of time. As a result, this device has a problem similar to that of the analog device.